1. Field of the Invention
The present invention relates to single phase permanently split capacitor (PSC) motor control circuits and more particularly to a variable speed motor control circuit having a bypass circuit which independently controls the auxiliary winding circuit of the PSC motor.
2. Discussion of the Background
Single phase PSC single and multiple speed motors are generally designed to operate at one or several specific speeds, respectively. However, it may sometimes be necessary to provide a variable speed capability for motors of this type, in order to allow motor operation at any speed over a range between minimum and maximum speed levels. Accordingly, a speed control may be coupled to the motor voltage source to selectively “chop” or phase proportion the voltage wave form. Referring to FIG. 1, an operating point other than synchronous speed or “full speed” of a motor control circuit configured to vary a motor speed is shown. The phase proportioning may be carried out by means of a triac or other component, operated as a switch, to pass only a portion of each wave form to the motor as shown by FIG. 1. The wave form portion can be varied to controllably vary a motor speed, by changing the time at which the switch is closed.
Referring to FIG. 2, a block diagram of a conventional arrangement of a motor control circuit configured to vary a motor speed is shown. In this configuration, a speed control 10 of the above type is coupled to a voltage source, such as a line L2, of a motor 12. However, either L1 or L2 may be configured as neutral, but not simultaneously. In this configuration, motor 12 necessarily comprises a single or multiple speed permanent split capacitor (PSC) motor, and is not applicable to shaded pole motors. This configuration also does not apply to a motor of a type in which the auxiliary winding is switched out of series with the start capacitor by mechanical means, such as a centrifugal switch or the like, commonly referred to as a “split phase” motor. Motor 12 is configured with an auxiliary winding 14 that is generally designed to adequately provide torque for initial motor start up and to improve the efficiency of the phase angle of the motor at full speed. After start up, a run winding 16 provides continuous motor operation. A capacitor 18 is shown connected in series with auxiliary winding 14, to ensure the motor rotates in the proper direction as it starts, and also to provide additional torque for start up. When the motor reaches its intended speed, the impedance in the capacitor 18 becomes high, thereby causing most of the motor current to flow through the run winding 16 and not the auxiliary winding 14. Thereafter, as the motor is being driven by run winding 16, speed control 10 is operated to vary motor speed as previously described.
Unfortunately, in a conventional arrangement, as shown in FIG. 2, the chopped voltage is applied to auxiliary winding 14 and capacitor 18 as well as to run winding 16. This results in loss of efficiency and harmonic distortion of the motor voltage, which in turn results in vibration, noise, and excessive heating of the motor.